Optical transmission systems for optical communications typically include a pair of network nodes connected by an optical waveguide (i.e., fiber) link. Within each network node, optical signals are converted into electrical signals for signal regeneration and/or routing. Exemplary network nodes of this type include Add-Drop-Multiplexers (ADMs), routers, and cross-connects. The optical link between the network nodes is typically made up of multiple concatenated optical components, including two or more (and possibly 20 or more) optical fiber spans (e.g., of 40-60 km in length) interconnected by optical (e.g., Erbium) amplifiers.
The use of concatenated optical components within the link enables improved signal reach (i.e., the distance that an optical signal can be conveyed before being reconverted into electrical form for regeneration). Thus, for example, optical signals are progressively attenuated as they propagate through a span, and amplified by an optical amplifier prior to being launched into the next adjoining span. However, each optical component exhibits polarization dependent effects, which may be manifested as either polarization dependent loss (in the case of filters, isolators, and fiber), or polarization dependent gain (in the case of optical amplifiers). Within discrete optical components such as filters, isolators and amplifiers, the polarization dependent effects are typically a function of wavelength. Within fiber, polarization dependent losses are a function of wavelength, but may also vary with stress, bending radius, and vibration of the fiber.
When considering the effects of polarization dependent loss/gain on a signal, it is convenient to consider the PDE as a vector quantity, and this terminology is used herein. A more rigorous treatment of PDE is provided in “Polarized Light” (Edward Collett, ISDN 0-847-8729-3). When multiple optical components are concatenated to form a link, the polarization dependent effect exhibited by the resulting system is the vector sum of the polarization dependent effects introduced by each of the various components, transformed by the polarization coupling between successive elements of the link. Because the polarization dependent effect of fiber is affected by environmental conditions, the vector sum will tend to be a bounded statistical entity having a static and a dynamic components. The static component is environmentally insensitive, and can be compensated by appropriate tuning of optical detectors in the receiving node. However, the dynamic component is (possibly rapidly) time-varying, and manifests itself as transient noise in received optical signals. This transient noise degrades the signal-to-noise ratio, and thereby impairs the performance of the optical transmission system.
Various equipment is known for measuring polarization dependent effects in a laboratory. However, laboratory measurements can only be used as estimates of the PDE of installed network links, because it is very difficult to duplicate, in a laboratory, all of the factors affecting PDE in the installed system. Furthermore, installation of such laboratory equipment in installed network links is generally impractical.
A method and system for monitoring transients caused by polarization dependent effects is disclosed in United Kingdom Patent Application No. 2328572A, entitled “Detecting Transients In An Optical Transmission System”, which was published on Feb. 24, 1999. According to this method, transients are measured at a receiving end of a link, and compared to known features of causes of transient effects. Thus, for example, the rise-time, peak value and pulse shape of a signal transient can be analyzed and compared to a database of known transient features to estimate whether the detected transient is caused by, for example, mode hopping in an optical amplifier or vibration of a fiber (indicated by periodic fluctuations in signal polarization).
In principle, the methods of United Kingdom Patent Application No. 2328572A could be used to measure polarization dependent effects in an installed optical communications network. However, as the number of cascaded optical components within the optical transmission system increases, it becomes increasingly difficult to correctly distinguish transients due to polarization dependent effects from those caused by simple (i.e., non-polarization dependent) attenuation and gain.
Accordingly a reliable technique for measuring polarization dependent effects (i.e., gain or loss) in an installed optical communications network remains highly desirable.